Means for measuring the frequency of an alternating current.



C. E. HIATT.

MEANS FOR MEASURING THE FREQUENCY OF AN ALTERNATING CURRENT.

APPLICATION FILED PEB.21, 1911.

1,1 21,626. Patentd Dec. 22, 1914.

WITNESSES:

'lVheatstone bridge each UNITED STATES PATENT OFFICE.

CASSIUS E. HIATT, OF EAST ORANGE, NEW JERSEY, ASSIGNOR T WESTINGHOUSEELECTRIC & MANUFACTURING COMPANY, A CORPORATION OF PENNSYLVANIA.

MEANS FOR MEASURING THE FREQUENCY OF AN ALTER NATING CURRENT.

Specification of Letters Patent.

Patented Dec. 22, 1914.

Application filed February 21, 1911. Serial No. 610,100.

a method of, and apparatus for, determining, with a very high degree ofaccuracy, the frequencies of alternating currents andwhich shall beadapted to measure frequencies ranging from only a few cycles tohundreds of thousands per second.

The single figure of the accompanying drawing diagrammaticallyillustrates apparatus embodying and for practisingmy invention, whichapparatus comprises 'a of the arms 1-234 of which is composed of veryfine iron wire that is wound or coiled upon suitable supports, theopposite arms 1 and 2 of the bridge being preferably placed upon asupport -and the other pair of opposite arms 3 and 4 being preferablyplaced upon another support 6. The supports 5 and 6 may be, and suitablyare, composed of mica, though they may be of any other suitablecharacter, and the wires composing the arms of the Wheatstone bridge areapplied thereto in z1g-zag form so that adjacent portions thereof areseparated and .insulated from each other. The iron wires composing thearms of the bridge are of very small diameter in' order that no eddycurrents shall be produced therein when magnetized, and,

also in order that they may have very low terminal capacities wherebythe sensitiveness of the apparatus is increased.

A battery 7 is connected between the juncti on of the arms 2 and 3 andthe junction of the arms 1 and 4 of the bridge, and a mirrorgalvanometer 8 is connected between the remaining two junctions of thearms of the bridge, with a suitable shunt box 9 properly rela' edthereto. A resistance 10 is also included in the arm 4 of the bridge forthe 5.5 the bridge.

conditions,

The arms 1 and 2 of the \Vheatstone bridge are surrounded by a solenoid11 comprising a suitable number of convolutions of wire which are woundinductively, and the arms 3 and 4 of the bridge are surrounded byanother solenoid 12 comprising the same number of convolutions of wireas the solenoid l1 and in all respects like it, except that it is woundnon-inductively. The coils l1 and 12 may be connected, by means of adouble-throw switch 13, either to a source 14: of alternating current orto a. source 15 of direct current, the alternating current, of course,being that of which it is desired to measure the frequency. When thecoils 11 and 12 are connected to the source of alternating current, theamount of current traversing the same may be regulated by means of arheostat 16, and the said current is'thereby preferably maintainedconstant during the makingof a measurement.

In order that the amount of current traversing the. solenoids may bedefinitely known, as an assistance to maintaining it constant, suitablemeans for measuring the same may be included in the circuit therewith,such, for instance, as a thermo-ammeter 17 which is used in conjunctionwith a galvanometer 18, in a manner set forth in a co-pendingapplication Serial No. 522,515, filed by me on Oct. 14, 1909, though anyother suitable means may be employed instead thereof. The coils .11 and12 are adapted to be connected to a source of direct current for thepurpose of establishing substantially steady and uniform temperatureconditions in preparation for a test, thereby eliminating possiblethermoelectric disturbances due to sudden heating of the coils andbridge arms when measurements are taken. Direct current may also be usedfor purposes of calibration, in which connection an ammeter'19-and aresistance 20 will be found useful. In order that the solenoids 11 and12 and the bridge arms may not be fl'ected by external temperature theyare preferably inclosed in a suitable heat-insulating casing 21.

In making use of the device, a balance ofthe Wheatstone bridge is firstsecured. The alternating current, the frequency of which it is desiredto measure, is then passed through the solenoids 11 and 12 andmaintained, as nearly as possible, constant in value. The arms 1 and 2ofthe bridge then become magnetized by the solenoid 11,

which. isinductively wound, and-are accordingly heated by the hysteresisthat is produced by the alternations of the flux therein,

the degree of heating, of course, being directly. proportional to thefrequency of alternations of the flux and the energizing current sinceeddy currents are eliminated by reason of the fineness of the wireconstiwound; The only heating effect, therefore,

to which the arms 3. and 4 are subjected is 1 a rheostatic or PR effectwhich is'exactly like the corresponding effect to which the arms 1 and-2are subjected by the solenoid 11. The degree of unbalancing of theWheatsto'ne bridge is then due only to the hysteretic heating effect ofthe energizing. alternating current upon the arms 1 and 2, which heatingeffect is directly proportional to the frequency of alternations of theenergizing current and may be measured by.

means of the galvanometer 8.

The apparatus is highly sensitive and permits of a determination of thefrequency of the alternations of a current with great accuracy. Theapparatus may also be utilized for measuring frequencies differingwidely in value, such as from 10 to'lO cyclesper second, but it isparticularly useful for the measurement of very high frequencies whichother instruments are" not adapted to measure. Y

I In adapting the apparatus for the measurement of comparatively lowfrequencies, the Wheatstone bridge may be balanced wlth direct currenttraversing the solenoids 11 and 12, but, in order to adapt it to measureextremely high frequencies, it will usually be found desirable to securea balance of the bridge with alternating current transversing thesolenoids 11- and 12 the frequency of which is less than that which is vto be measured.

I claim as my invention:

1. A frequency meter comprising a Wheatstone bridge the arms of whichare formed of fine iron wire, an inductively wound coil surrounding.-two opposite arms of the Wheatstone bridge, and a similarnon-inductively woundcoil surrounding'the other two opposite arms of thesaid bridge.

2. A frequency meter comprising a Wheatstone bridge the arms of whichare formed of fine iron wire, an inductively wound coil surrounding twoopposite arms of the Wheatstone bridge, and a similar non-inductivelywound coil surrounding the other two opposite arms of the said bridgeand connected in series with the aforesaid coil.

3. A frequency meter comprising a VVheatstone bridge the arms of whichare formed of fine iron wire, an inductively wound coil surrounding twoopposite arms of the "Wheatstone bridge, a similar non-inducformeasuring any unbalancing of the resistances respectively of theinductively and non-inductively surrounded legs of the -Wheatstonebridge.

5. A frequency meter comprising a Wheatstone bridge the arms of whichare formed of fine iron wire, an inductively wound coil surrounding twoopposite arms of the lVheatstone bridge, and a similar non-inductivelywound coil surrounding the other two opposite arms of the said bridge,and a heat-insulating casing. surrounding the said bridge and coils.

In testimony whereof, I have hereunto subscribed my name thistenth dayof February, 1911.

CASSIUS E. HIATT. Witnesses:

GEORGE P. SoHoLL,

A. YOUNGHOLM.

